The present invention also relates to a new process for preparing these metallic effect pigments of the invention and also to their use in paints, printing inks, varnishes, electrodeposition coating materials, powder coating materials, plastics, and cosmetics.
The optical effect of metallic effect pigments is based on the directed reflection of light from platelet-shaped pigment particles in parallel alignment. The luminance, and occasionally the hue as well, change with the viewing angle. Characteristic of metallic effect pigments are their metallic luster and their outstanding opacity. Important for the visual appearance are optimum distribution and also sufficient chemical stability of the metallic effect pigments in the application medium.
Disadvantageously, metallic effect pigments are of limited stability in an aqueous medium. Aluminum pigments, for example, exhibit comparatively rapid decomposition in water, forming hydrogen and aluminum hydroxide. To prevent this, the surfaces of the metallic pigment are conventionally protected by phosphating, chromating or silanizing. Becoming increasingly significant, in addition, are synthetic resin-coated metallic effect pigments, on account of their nonconductivity and their better compatibility in coating systems.
EP 0 477 433 B1 describes synthetic resin-coated, aluminum-based metallic pigments in which a layer of synthetic resin is attached covalently via a siloxane coating applied to the pigment surface. This siloxane layer is intended, as an adhesion promoter, to ensure effective attachment of the synthetic resin coat on the pigment surface. It has nevertheless emerged that, under the action of shearing forces, there may be partial detachment of the synthetic resin layer. Over a relatively long period of time, therefore, these pigments are not reliably stable toward gassing and storage.
EP 0 280 749 B1 discloses resin-coated metallic pigments where between the pigment surface and the layer of synthetic resin there is an ethylenically unsaturated carboxylic acid and/or a phosphoric monoester or diester as an adhesion promoter. The carboxyl groups of the carboxylic acid monomer and/or the phosphate groups of the phosphate monomer attach to the metallic pigment surface. The ethylenic double bonds thus arranged at the metal pigment surface are used for reaction with further monomers, to form a highly crosslinked synthetic resin layer. In spite of this three-dimensional structure to the synthetic resin coat, the gassing stability of these metallic pigments is low.
WO 2005/063897 A2 discloses chemically and mechanically stable metallic effect pigments which are coated with oligomeric and/or polymeric binders that are crosslinkable chemically and/or on exposure, for example, to UV or IR radiation. The metallic effect pigments can in this way be embedded in a polymeric film. After the metallic pigments have been coated, the binders are still curable or polymerizable, which is why the metallic pigments find use in powder coating material. In the course of coating and/or in the course of evaporation of the solvent, it is true that the binders may undergo some slight incipient polymerization, but they do not cure. Precoating the pigment surface with functionalized silanes and/or polymers or organophosphorus compounds improves the adhesion of the binder envelope.
Known from EP 1 084 198 B1 are effect pigments whose surface has been modified with orientation aids. The orientation aid, present in monomeric or polymeric form, carries at least two different functional groups which are separated from one another by a spacer. One of the functional groups is chemically attached to the pigment; the other is able to react, for example, with the binder of a surrounding coating material in a kind of crosslinking reaction, and hence is able to stabilize the pigment with nonleafing behavior. This produces effect pigments which on the one hand are readily wetted by the binder and/or solvent of the paint or varnish, and can be oriented effectively in the liquid coating film, and on the other hand enter into an intimate bond with the surrounding binder matrix. The condensation resistance of a cured coating material can be improved considerably in this way.
U.S. Pat. No. 4,213,886 discloses a coated aluminum pigment where, in a first layer, a silane with a monoethylene group and, subsequently, an acrylate resin layer is applied. It has emerged that this two-layer structure comprising silane layer with subsequent acrylate layer is not sufficiently impervious to penetration of water and chemicals.
Furthermore, DE 10 2005 037 611 describes metallic effect pigments having an inorganic/organic hybrid layer and possessing not only high mechanical stability but also good gassing stability. This hybrid layer has organic oligomers and/or polymers with an inorganic network consisting of inorganic oxide component(s) that are connected at least partly covalently via network formers. The network formers may be organofunctional silanes.
DE 10 2004 006 145 A1 discloses platelet-shaped substrates having a functional multilayer structure, composed of one or more layers of a polymer and one or more layers of a silane. The layers of one or more silanes in this arrangement exert a barrier function, and the one or more polymer layers stabilize the particles with respect to agglomeration and mechanical influences.